1. Field of the Invention
This invention relates to subsea control systems and more particularly to systems for positive control of underwater Christmas tree operations.
2. Description of the Prior Art
The production of oil and gas from offshore wells is well known in the petroleum industry. Wells are commonly drilled several hundred or even several thousand feet below the surface of the ocean. These wells must not only be drilled without the use of divers at the greater depths, but the connecting, testing and servicing of pipes and the operation of regulating valves must be performed on these wells during and after the drilling process.
It is well known to use surface controlled Christmas trees mounted at the wellhead of these undersea wells to control production in such wells. Such Christmas trees include specific-function operating valves which control the actual flow of oil from the well through a flowline to a storage device at the ocean surface. These Christmas tree valves can be controlled by hydraulic control systems, by electrical systems or by a combination of electrical and hydraulic control systems. In any of these systems it is desirable to use as few control lines as possible between a surface control center and the Christmas tree.
It is also desirable that a back-up control system be provided in case of a failure of the primary control system. Any failure of a single control system would be expensive because of lost production and because of the expense of the equipment needed to do the repairs. A back-up system can prevent shutdown of production and may allow repairs to be made at a more convenient time. The back-up system generally extends the usable life of the control system.
One such dual control system is shown in the U.S. Pat. No. 3,894,560 to Baugh. However, the shuttle valves which control the valve operators are enclosed in the tree cap of the Baugh apparatus. Thus, the tree cap must be removed and the tree opened to expose the working parts of the tree to sea water and corrosion in the well in order to replace these valves. Also the Baugh apparatus includes a single control module.
What is needed is a control system wherein the primary control module can be replaced while the back-up control module continues to operate the Christmas tree valves. It would also be advantageous to position the shuttle valves so that they could be replaced without removal of the tree cap.